FIG. 5(a) is a perspective view showing a structure of the prior art resistor film heating-type variable wavelength semiconductor laser disclosed in Applied Physics, Spring Meeting 1991, Prescript Number 3, page 969, 29p-D-8. In FIG. 5(a), reference numeral 101 designates an n-type semiconductor substrate. A stripe-shaped active layer 102 is disposed on the substrate 101 at the central portion of the laser element. A semi-insulating current blocking layer 103 is disposed on the substrate 101 at both sides of the active layer 102. A p-type semiconductor layer 104 is disposed on the current blocking layer 103 and the active layer 102. A p side electrode 105 and an n side electrode 106 for injecting a laser driving current are disposed on the front surface of the p-type semiconductor layer 104 and on the rear surface of the substrate 101, respectively. An insulating film 107 is disposed directly opposite the active layer 102 on the p side electrode 105 and a platinum resistor film 108 is disposed on the insulating film 107 at a position directly opposite the active layer 102.
When a bias voltage in a forward direction is applied to the p-n junction of the semiconductor laser through the p side electrode 105 and the n side electrode 106, charge carriers are injected into the active layer 102 and recombine in the active layer 102 to generate light. Here, the current blocking layer 103 is provided so that current is concentrated in the active layer 102. The light generated in the active layer 102 is guided along the active layer with repeated reflection and amplification, thereby producing laser oscillation.
The oscillation wavelength of a semiconductor laser varies with the temperature of the active layer and when the temperature rises, the wavelength is generally lengthened because the band gap energy of the semiconductor of the active layer is narrowed, thereby shifting the wavelength to the longer wave-length side. The variable wavelength semiconductor laser of FIG. 5(a) utilizes this effect. In that laser device, the resistor film 108 generates heat when a current flows between the ends 108a and 108b of the resistor film 108, thereby heating the active layer 102. Therefore, when the current flowing through the resistor film 108 is increased, the temperature of the active layer 102 rises and the oscillation wavelength is shifted to the longer wavelength side, as shown in FIG. 5(b).
FIG. 6 is a perspective view of a prior art variable wavelength semiconductor laser disclosed in Japanese Published Patent Application 1-173686. In FIG. 6, reference numeral 111 designates an n-type GaAs substrate. An n-type GaAs buffer layer 112 is disposed on the substrate 111, an n-type AlGaAs cladding layer 113 is disposed on the buffer layer 112, a GaAs active layer 115 is disposed on the cladding layer 113, a p-type AlGaAs cladding layer 117 is disposed on the active layer 115, and a p-type GaAs cap layer is disposed on the cladding layer 117. A stripe-shaped laser driving electrode of width w.sub.d 121 is disposed on the cap layer 118 at a central portion of the laser element and temperature control electrodes of width w.sub.m (w.sub.m is significantly larger than w.sub.d) is disposed on the cap layer 118 on both sides of and separated from the laser driving electrode 121. In the semiconductor laminated structures at regions between the laser driving electrode and the temperature control electrodes 122, respectively, ions, such as He ions, are implanted from the surface of the cap layer 118, reaching the semiconductor substrate 111 and producing high resistance regions 119. A common electrode 123 is disposed at the rear surface of the substrate 111. Reference numeral 125 designates a laser active region.
In this prior art device, laser oscillation occurs at the region 125 of the active layer 115 directly opposite the driving electrode 121 from an excitation current injected from the driving electrode 121. Further, since a region 125 of the active layer 115 directly opposite the temperature control electrode 122 has a large width, laser oscillation is not likely to occur there even if a current flows, as shown by arrows A. By making relatively large currents flow from the electrodes 122, the regions directly opposite the electrodes 122 can be used as heating materials. Accordingly, by controlling the temperature of the laser active region 125 directly opposite the driving electrode 121 for the active layer 115 with the heat generated at regions directly opposite the electrode 122, the laser oscillation wavelength can be controlled.
FIG. 7 is a cross-sectional view of another prior art variable wavelength semiconductor laser disclosed in Japanese Published Patent Application 1-173686. In FIG. 7, reference numeral 131 designates an n-type GaAs substrate. An n-type GaAs buffer layer 132 is disposed on the substrate 131, an n-type AlGaAs cladding layer 133 is disposed on the buffer layer 132, an n-type AlGaAs-GaAs graded layer 134 is disposed on the cladding layer 133, a GaAs active layer 135 is disposed on the graded layer 134, a p-type GaAs-AlGaAs graded layer 136 is disposed on the active layer 125, a p-type AlGaAs cladding layer 137 is disposed on the graded layer 136, and a p-type GaAs cap layer 138 is disposed on the cladding layer 137. A stripe-shaped laser driving electrode 141 of width w.sub.d is disposed on the cap layer 138 at a central portion of the laser element, and temperature control electrodes 142 of width w.sub.m (w.sub.m is significantly larger than w.sub.d) are disposed on the cap layer 138 at both sides of the laser driving electrode 141. Grooves extending from the surface of the cap layer 138 to the cladding layer 137 are produced by etching where the laser driving electrode 141 is disposed and where the temperature control electrodes 142 are disposed, respectively, and the grooves are filled with insulating layers 139 comprising polyimide or the like. A common electrode 143 is disposed at the rear surface of the substrate 131. Reference numeral 145 designates a laser active region.
Also in this prior art device, similar to the semiconductor laser of FIG. 6, laser oscillation occurs at a region 145 directly opposite the driving electrode 121 of the active layer 135 from an excitation current injected from the driving electrode 141. Further, since the regions directly opposite the temperature control electrodes 142 of the active layer 135 have a large width, even when a current flows, as shown by the arrows B, from the electrode 142, laser oscillation is not likely to occur. A relatively large current flow from the electrode 142 through the regions directly below the electrode 142 can be used as a heat source. By controlling the temperature of the laser active region 145 of the active layer 135 directly opposite the driving electrode 141 with the heat generated at regions directly opposite the electrode 142, the oscillation wave-length of the laser can be controlled.
FIG. 8 is a partly broken away perspective view of a prior art variable wavelength semiconductor laser disclosed in Japanese Published Patent Application 1-173686. In FIG. 8, reference numeral 151 designates a p-type GaAs substrate. A p-type GaAs buffer layer 152 is disposed on the substrate 151, a p-type AlGaAs cladding layer 153 is disposed on the buffer layer 152, a GaAs active layer 155 is disposed on the cladding layer 153, an n-type AlGaAs cladding layer 157 is disposed on the active layer 155, and a p-type GaAs cap layer 158 is disposed on the cladding layer 157. The laminated structure from the cap layer 158 to the buffer layer 152 is formed into a stripe-shaped ridge configuration, and insulating layers 159 are disposed on the substrate 151 at both sides of the ridge. A stripe-shaped resistance layer 164 is disposed parallel to the ridge in the insulating layer 159. An insulating film 165 is disposed covering the insulating layer 159 and the resistance layer 164. This insulating film 165 has apertures at both ends of the resistance layer 164 and temperature control electrodes 162 are disposed on the resistance layer 164 exposed in the apertures. An n side electrode 161 for driving the laser is disposed in contact with the stripe-shaped cap layer 158. Further, a p side electrode 163 is disposed on the rear surface of the substrate 151.
In this prior art laser device, laser oscillation occurs at the active layer 155 from an excitation current injected from the driving electrode 161. Further, by making a current flow through the resistance layer 164 through the electrode 162, as shown by arrows C in the figure, the resistance layer 164 generates heat, the temperature of the active layer 165 is changed, and the laser oscillation wavelength is controlled. The striped-shaped resistance layer 164 is produced in the insulating layer 159 by ion implantation and the resistance value of the insulating layer 159 can be arbitrarily determined by the ion implantation.
The prior art resistance film heating-type variable wavelength laser shown in FIG. 5(a) requires producing an insulating film 107 on the p side electrode 105 and producing a platinum resistance film 108 thereon, resulting in high production costs. In addition, the heat conductivity is inferior because there exists an insulating film 107 between the resistance film 108 and the active layer 102 and, even when the platinum resistance film is heated by a current flow, it takes time for the heat to be transmitted to the active layer, resulting in a delayed change of wavelength in response to the wavelength controlling current.
The variable wavelength lasers of FIGS. 6 and 7 heat a part of a grown layer and require no complicated structure for producing a particular region to be heated. However, the regions to be heated are separated by ten to several tens of microns from the active region, thereby taking time for the transmission of heat to the active region, also delaying a change in wavelength in response to the wavelength controlling current.
Further, the variable wavelength laser of FIG. 8 requires an ion implantation process for producing the resistance layer, and that process is complicated. As in the other prior art devices, the region to be heated is separated by ten to several tens of microns from the active region, thereby requiring time for the heat of the region to be transmitted to the active region, resulting in a delayed change of wavelength in response to wavelength controlling current.